Microwave oscillator



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L- M D 40 7% I I 5 I I ATTORNE Y5 United States Patent 3,260,961MICROWAVE OSCILLATOR Burton J. Udelson, Bethesda, Md., assignor to theUnited States of America as represented by the Secretary of the ArmyFiled Jan. 13, 1965, Ser. No. 425,366 6 Claims. (Cl. 331-81) Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment to me of any royalty thereon.

This invention relates generally to electron beam devices and moreparticularly to a DC pumped voltage tunable oscillator.

One of the more important advances made in the microwave art in recentyears is the development of practical parametric devices. A parametricdevice may be generally considered to be a mechanism in which aparticular output function is achieved through the periodic variation ofsystem parameter. As applied to high frequency electron beam devices,the term generally refers to a device in which a variation in theelectron beam is amplified through the periodic variations of certainelectron beam or circuit parameters by means of a pump.

An early successful embodiment of the principles of parametricamplification applied to an electron beam device, was the quadrupoleamplifier. This amplifier includes an electron gun for emitting a streamof electrons, means for magnetically focusing the electron beam, aninput coupler of the resonant or Cuccia type, a quadrupole interactionregion, an output coupler, and a collector. While highly satisfactoryfor many applications, this type of parametric amplifier has severallimitations. One of the chief limitations is that the pump energy, usedto supply energy to the electron beam and produce amplification, must betwice the signal frequency. As the signal frequency increases, thedisadvantage resulting from the requirement that the pump operate at afrequency twice the signal frequency becomes more and more acute.

To overcome this disadvantage, especially apparent at the higher signalfrequencies, several electron beam parametric amplifiers With low pumpfrequencies or with DC. pumps have been developed. D.C. pumped devicesare characterized by having a DO pumped section which acts to increase atransverse displacement in the electron beam at the expense ofdecreasing the longitudinal velocity of the beam. All D. C. pumps may beclassified into three basic types depending upon the wave interactionthat takes place. The three types of wave interaction are: couplingbetween a fast and a slow cyclotron wave; coupling means a fast or slowcyclotron wave and a synchronous wave; and coupling between twosychronous waves.

These different types of DC. pumps may be distinguishedby reference tothe general formula for the condition of DC. pumping:

where n is an integer, L is the spacial periodicity of the change of anyof the parameters of the system, and A is the wavelength due to thenatural resonant frequency of the system. Coupling between a fast and aslow cyclotron wave is characterized by 11: 1. Coupling between a fastor slow cyclotron wave and a synchronous wave is characterized by n= 2.Coupling between the synchronous waves is a special case of n=1 whenboth L and A are equal to infinity.

Several prior art devices have made use of these D.C. pumpingprinciples. Usually the required natural resonance of the system is thecyclotron resonance due to the longitudinal magnetic focusing field, andspacial periodicity is achieved either by varying periodically thevoltage 3,260,951 Patented July 12, 1966 'ice along the length of thepumping section or by varying periodically the strength of the magneticfield. While such devices have obviated the need for higher pumpfrequencies', the use of a magnetic field makes these devices bulky,heavy, and sensitive to both temperature variation and shock.Furthermore, magnetically focused electron beam devices can havedifferent values of natural resonant frequency only by introducingadditional pole pieces.

In a separate application Serial No. 289,762, filed June 21, 1963, nowPatent No. 3,205,449, and assigned to the assignee of the instantapplication, the applicant has disclosed a novel D.C. pumpedelectrostatically focused electron beam parametric amplifier which hasno magnetic field. The two functions of the magnetic field in the priorart devices, namely, the focusing of the electron beam and the creatingof a cyclotron frequency, are accomplished by electrostatic focusinglenses. The applicant has discovered that by the use of certainingenious structure the principles of operation of the amplifierdisclosed in his application Serial No. 289,762 may be employed in DC.pumped electrostatically focused electron beam microwave oscillatorhaving many advantages over prior art microwave oscillators.

It is therefore an object of this invention to provide an electron beammicrowave oscillator that does not require a magnetic field for focusingthe electron beam or establishing a natural resonant frequency for thesystem.

It is another object of the invention to provide a microwave oscillatorwhich is light in weight, rugged, stable over a wide temperature range,and easy to construct.

It is a further object of the invention to provide a DC. pumpedelectrostatically focused electron beam microwave oscillator which isvoltage-tunable over a broad range of frequencies.

According to the present invention, the foregoing and other objects areattained by providing an electron gun which develops a sheet electronbeam, a collector longitudinally spaced from the electron gun andpositioned to receive an electron sheet beam developed by the electrongun, and an electrostatically focused D.C. pump interaction structurepositioned between the electron gun and the collector and having aplurality of electrostatic focusing lenses periodically spacedlongitudinally along the path of a sheet electron beam developed betweenthe electron gun and the collector, the lens strength of theelectrostatic focusing lenses being above the critical value at which asheet electron beam developed between the electron gun and the collectorbecomes unstable causing the sheet electron beam to have a naturalfrequency of transverse oscillation that increases in amplitudeexponentially about a central plane extending longitudinally between theelectron gun and the collector, the interaction structure beingelectrically divided into two regions to cause the R.F. field in theinteraction structure adjacent the electron gun to be out of phase withthe R.F. field in the interaction structure adjacent the collector.

The specific nature of the invention, as well as other objects, aspects,uses and advantages thereof, will clearly appear from the followingdescription and from the accompanying drawing in which:

FIG. 1 is illustrative of the DC. pumping action employed in thisinvention;

FIG. 2 shows a specific embodiment of the invention wherein shortingbars are used to establish a standing Wave of transverse R.F. electricfield thereby providing the required 180 phase reversal;

FIG. 2a illustrates the phase reversal of the R.F. electric vector inthe structure shown in FIGURE 2;

FIG. 3 shows another specific embodiment of the invention wherein therequired 180 phase reversal is provided by crossing over electrostaticfocusing lens connections in the interaction structure; and

FIG. 3a illustrates the phase reversal of the RF. electric vector in thestructure shown in FIGURE 3.

Referring now to the drawings and more particularly to FIGURE 1, aseries of deflector plate pairs 11-11 and 12-12 are alternately andperiodically spaced longitudinally along the path of a sheet electronbeam, represented by the line 14. A first voltage V is applied to thedeflector plate pairs 12-12, and a second voltage V is applied to thedeflector plate pairs 11-11. In FIG- URE l the voltage V is greater thanthe voltage V and the arrows between plate pairs 11-11 and 12-12represent the electrostatic forces acting on electrons. The deflectorplate pairs 11-11 and 12-12 are maintained at potentials positive withrespect to the cathode of the electron gun (not shown) which producesthe sheet electron beam represented by line 14. The potential differencebetween deflector plate pairs 11-11 and 12-12 establishes a series ofconvergent electrostatic lenses along the direction of electron beamtravel. The electrostatic focusing lens system imparts to the electronsheet beam a time average restoring force that acts to pull the beamtowards the plane of symmetry represented by the line 15. This restoringforce establishes a transverse natural resonant frequency and acorresponding transverse resonant electron beam wavelength. Thiselectron wavelength decreases with increasing lens strength. Lensstrength is proportional to When the lens strength has been increased tothe point where the electron wavelength is equal to two periods DC.voltage variation (A =2L, where L is the spacial periodicity of voltagevariation), instability of the electron beam motion occurs, and thetransverse displace ment of the electron beam increases exponentiallywith distance at the expense of decreasing longitudinal velocity of theelectron beam. Solutions of the paraxial-ray equation have shown thatthroughout this unstable region the electron wavelength remains fixed attwo periods of DC. voltage spacial variation, and that the phase of thetransverse displacement of the exponentially increasing beam is suchthat the beam always crosses the plane of symmetry in the middle of ahigh potential plate pair 12-12 and has its maximum transversedisplacement in the middle of a low potential plate pair 11-11. Thisinstability is caused by a form of parametric pumping, i.e., eachelectron experiences a rate of change of DC. electric field at twice itsnatural resonant frequency, and has been termed D.C. pumping because noexternal R.F. pumping field is applied. By proper choice of the ratio ofa/d where d is the transverse spacing between plates and a is thelongitudinal periodicity of plate pairs, the desired instabilitycondition can be achieved with the voltage V made equal to the cathodeor ground potential. The electron gun can be designed so that therequired current is obtained with the electron gun anode operated at avoltage equal to V Thus, the oscillators according to the presentinvention require only one source of DC. potential. Setting the voltageV equal to zero also creates the favorable situation in which thedesired condition of electron beam instability is unaifected by varyingV thereby facilitating tuning the oscillators. If V were a voltage otherthan zero, tuning the oscillators would require both the voltages V andV to be varied simultaneously to maintain the desired condition ofinstability. Setting the anode voltage equal to V also assures that therequired current enters the structure at all times. This is mostimportant because for an electron beam operating in the unstable regionthe beam thickness will increase exponentially due to instability of theelectrons within the beam if the current in the structure is too high ortoo low. An approximate formula for the required perveance of anelectron sheet beam in the unstable region of an electrostatic focusingstructure is condition of instability. Setting the anode voltage equaland V is the space average voltage and is equal to The electron beamalso has a tendency to spread in the lateral direction. An electrostaticfield that acts to counteract the space charge spreading forces in thislateral direction can be created by curving the plates at the voltage Vin a convex manner and making the plates at the voltage V planar. Thistechnique of lateral focusing is described in more detail in another ofthe applicants patent applications Serial No. 207,130, filed July 2,1962, and assigned to the assignee of the instant application. Besidesacting to focus the electron beam laterally, this technique also acts tocounteract the tendency of the space charge depression at the center ofthe electron beam to create a variation of longitudinal electronvelocity as a function of lateral position. This mechanism of lateralfocusing is of particular interest for the oscillators of the instantapplication where the perveance of the plate structure is kept constantbecause the lateral focusing force counterbalances the space chargespreading force for all values of the voltage V The particular platestructure shown in FIGURE 1 is only illustrative of the structure thatmay be employed in the oscillators of this invention. Other anddifferent plate structures may be used with equal effect. Exemplary ofsuitable plate structures are those described and shown in the aforesaidapplication Serial No. 289,762 with particular reference to FIGURES 3,4, 5 and 6 of the said application.

Reference is now made to FIGURE 2 of the drawings which shows a specificembodiment of a microwave oscillator according to the invention.Basically, the oscillator comprises an electron gun 21 for producing asheet elec tron beam, a collector 22 for receiving the electron beam,and an interaction structure having two regions 23 and 24 disposedbetween the electron gun 21 and the collector 22. The interactionstructure comprises a series of deflector plate pairs 25-25 and 2 6-26substantially as described with reference to FIGURE 1. Deflector platepairs 25-25 are connected to a source of voltage V 27 through an RFchoke 28, and deflector plates pairs 26-26 are connected to a source ofvoltage V 29 through an RF. choke 3 1. Adjacent plates having differentD.C. potentials are made to have the same R.F. potential by means ofcapacitors 3-2, 33, 34 and 35 which are connected between deflectorplates 25 and 26, and 25 and 26. Distributed inductances 36 and 37 areconnected to deflector plates to create a resonant L-C circuit. Thus, itcan be seen that the plates of the interaction structure are connectedto constitute a parallel plate transmission line. In addition, shortingbars 38 and 3 9 are connected across opposing plates of two pairs ofadjacent deflector plates pairs near the electron gun 2 1. Shorting bars38 and 39 divide the interaction structure into two regions, theinteraction structure to the left of the shorting bars being region 23and that to the right being region 24. The RF. short circuit made by theshorting bars 38 and 39 establishes a standing wave of transverse R.F.electric field as illustrated in FIGURE 2a. As indicated by FIGURE 2a,the total length of the interaction structure shown in FIGURE 2 issomewhat greater than a quarter wavelength long, the length of region 24being approximately equal to one quarter-wavelength. The standing waveestablished by the shorting bars assures that the RF. electric field atthe electron gun end of the structure is out of phase with the RF.electric field at the collector end of the structure.

' The required phase reversal of R.F. electric field can be achieved ina structure that is substantially shorter than one quarter-wavelengthlong as shown in FIGURE 3. Except for the diiference in length of theinteraction structure, the oscillator shown in FIGURE 3 is substantiallythe same as the oscillator shown in FIGURE 2. Again, the intersectionstructure is divided into two regions 41 and 42; however, in this casethe division, and the required 180- phase reversal of the R.F. electricfield is achieved by crossing over the deflector plate connections. Moreparticularly, opposing plates of a deflector plate pair at potential Vare cross-connected with opposing plates of the next succeedingdeflector plate pair at potential V while opposing plates of a deflectorplate pair at potential V adjacent to the first mentioned deflectorplate pair 'at potential V are cross-connected with opposing plates ofthe next succeeding deflector plate pair I at potential V The phasereversal of the R.F. electric field resulting from thesecross-connections is illustrated in FIGURE 3a.

j The operation of the oscillators shown in FIGURES 2 and 3 is the same.Electrons entering the interaction structure have a transverse motionimparted to them by the transverse R.F. electric field. As soon as theelectrons have a transverse displacement, the D.C. pumping mechanism dueto the condition of electron beam instability in the electrostaticfocusing structure also acts on the electron beam. Thus, in the regionof the interaction structure adjacent to the electron gun (region 2-3 inFIG- URE 2 and region 41 in FIGURE 3), both the R.F. field and the D.C.pumping mechanism act to enhance the transverse displacement of theelectron sheet beam causing all the electrons throughout this region tobe displaced on the same side of the plane of symmetry of theinteraction structure. This displacement defines an electron 'beam Wavewhich oscillates back and forth across the plane of symmetry at thenatural resonant frequency of the beam. When the electron beam passesfrom the region of the interaction structure adjacent to the electrongun to the region adjacent the collector (region 24 in FIGURE 2 andregion 42 in FIGURE 3), the phase relationship between the electron beamwave and the R.F. electric field changes by 180. As a result, transverseenergy is now transferred from the electron beam to the R.F. field. Inthis region, the D.C. pumping mechanism continues to enhance thetransverse electron beam motion, While the interaction of the electronbeam with the R.F. field acts to decrease the transverse beam motion. Byproper design of the device, i.e., by choosing the proper value for thefocusing lens strength determined by the ratio d/a and also the propervalue for the lengths of the two regions which comprise the interactionstructure, the R.F. field will remove transverse beam energy as fast orfaster than the beam obtains energy from the D.C. pumping mechanismthereby preventing electrons in the proper phase from impinging on thedeflector plates. Most of the energy coupled onto the plates in theregion of the interaction structure adjacent the collector is coupled tothe R.F. output. Some of the energy is dissipated in the LC tankcircuit, and some is utilized as feedback by having the R.F. fieldimpart a transverse motion of the proper phase to the electron beam inthe region of the interaction structure adjacent the electron gun.

The output frequency of the oscillators according to this invention isequal to the natural resonant frequency of the electron beam. Since theelectron beam wavelength is constant \e=2L) when the focusing structureis operating in the unstable region, the natural resonant frequencydepends only on the time average velocity (V,,,, of the electron beamwhich is a function of the voltage V Thus,

ft: A, 2L

where K is a constant proportional to the ratio of V V The value of Kdepends on the geometry of the focusing structure and the ratio of V /VThe upper limiting frequency of the oscillator depends on how small thedeflecting plate periodicity, a, can be made and how large V can bemade. For example, assuming a equal to 30 mils and V equal to 2,000volts, the upper frequency of operation is about 5500 mc. It is possibleto tune the oscillators according to the invention over a range ofapproximately 10% of a center frequency of operation by varying thedeflector plate voltage V The oscillators can be made to oscillate overa wider range of frequencies simply by changing distributed inductances.

Since frequency, structure periodicity, time average velocity, andcurrent are so closely related, the D.C. input power at a givenfrequency is determined uniquely by the structure periodicity a. Anapproximate formula for the D.C. input power is Since the beamcross-sectional area, A, can be made proportional to d, and d/a ischosen at a fixed value to make V equal to zero, a is proportional to A,and the input power is proportional to a Thus, the input power can bevaried over a fairly wide range. The lower value of input power islimited by either how small a can be made or the minimum currentrequired to sustain oscillations. The upper value of the input power islimited either by how high V can be made without causing inter-electrodearcing or how high large a current is available from the electron gun.For a given structure, assuming a constant efiiciency and a constantperveance for the device, the output power increases strongly withincreasing frequency as shown by the equation for power input.

The efliciency of the oscillators according to the invention is not highbecause of certain limiting factors. Depending upon the R.F. phase atwhich electrons are introduced into the interaction structure, someelectrons undergo large transverse displacements and correspondinglylarge longitudinal deceleration while others experience no transversedisplacement and no longitudinal deceleration. Those electrons that donot experience any longitudinal deceleration obviously play no part intransferring D.C. energy to R.F. energy and thus contribute to loweringthe overall efficiency. Furthermore, those electrons that are sloweddown by the D.C. pumping mechanism have their natural resonant frequencychanged due to the fact that their time average longitudinal velocityhas changed. Decelerated electrons will therefore fall out of therequired phase for optimum interaction with the R.F. field,deleteriously affecting the efliciency. These decelerated electronswill, however, impinge on the deflector plates and be removed from theinteraction region before falling more than out of phase with the R.F.field thereby preventing them from reabsorbing R.F. energy. The reasonfor this is that the decelerated electrons give up less transverseenergy to the R.F. field due to their falling out of phase with the R.F.field. In addition, they also experience a higher rate of exponentialtransverse displacement because the effective electrostatic lensstrength increases with decreasing electron velocity. It is possiblethat tapering the periodicity of the plate structure may enhanceefficiency in a similar manner to the improvements that have beenachieved in traveling- Wave tubes and backward-wave oscillators.

What have been described are D.C. pumped electrostatically focusedmicrowave oscillators having the advantages of being voltage tunable,simple to construct, extremely rugged, lightweight since they require nomagnet, and requiring only one D.C. voltage source. Obviously, manymodifications and variations of the present invention are possible inview of the above teachings. It is therefore, to be understood, thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

I claim:

1. A voltage-tunable D.C. pumped electrostatically focused electron beammicrowave oscillator comprising:

(a) an electron gun which develops a sheet electron beam,

(b) a collector longitudinally spaced from said electron gun andpositioned to receive a sheet electron beam developed by said electrongun, and

(c) an electrostatically focused D.C. pump interaction structurepositioned between said electron gun and said collector and having aplurality of electrostatic focusing lenses periodically spacedlongitudinally along the path of a sheet electron beam developed betweensaid electron gun and said collector, the lens strength of saidplurality of electrostatic focusing lenses being above the criticalvalue at which a sheet electron beam developed between said electron gunand said collector becomes unstable causing the sheet electron beam tohave a natural frequency of transverse oscillation that increases inamplitude exponentially about a central plane extending longitudinallybetween said electron gun and said collector, said interaction structurebeing electrically divided into two regions to cause the R.F. field inthe interaction structure adjacent said electron gun to be 180 out ofphase with the R.F. field in the interaction structure adjacent saidcollector.

2. A voltage-tunable D.C. pumped electrostatically focused electron beammicrowave oscillator as defined in claim 1 wherein said electrostaticfocusing lenses comprise:

(a) first and second series of deflector plate pairs alternately andperiodically spaced longitudinally along the path of a sheet electronbeam developed between said electron gun and said collector,

(b) a first source of positive voltage connected to said first series ofdeflector plate pairs, and

(c) a second source of voltage less than the voltage of said firstsource connected to said second series of deflector plate pairs.

3. A voltage-tunable D.C. pumped electrostatically focused electron beammicrowave oscillator as defined in claim 2 wherein said electron gunincludes a cathode and an anode and said first source of positivevoltage is further connected to said anode and said second source ofvoltage is further connected to said cathode.

4. A voltage-tunable D.C. pumped electrostatically focused electron beammicrowave oscillator as defined in claim 3 wherein said first source ofpositive voltage is variable.

5. A voltage-tunable D.C. pumped electrostatically focused electron beammicrowave oscillator as defined in claim 2 wherein opposing plates oftwo pairs of adjacent deflector plate pairs near said electron gun areR.F. short circuited thereby establishing a standing wave of trans verseR.F. field and thus defining the division of said inter-f actionstructure into two regions.

6. A voltage-tunable D.C. pumped electrostatically posing plates of thenext succeeding deflector plate pair.

of said second series thereby reversing the phase of the transverse R.F.field and thus defining the division of said interaction structure intotwo regions.

References Cited by the Applicant UNITED STATES PATENTS 3,148,302 9/1964Clavier et al.

ROY LAKE, Primary Examiner.

J. KOMINSKI, Assistant Examiner.

1. A VOLTAGE-TUNABLE D.C. PUMPED ELECTRONICALLY FOCUSED ELECTRON BEAMMICROWAVE OSCILLATOR COMPRISING: (A) AN ELECTRON GUN WHICH DEVELOPS ASHEET ELECTRON BEAM, (B) A COLLECTOR LONGITUDINAL SPACED FROM SAIDELECTRON GUN AND POSITIONED TO RECEIVE A SHEET ELECTRON BEAM DEVELOPEDBY SAID ELECTRON GUN, AND (C) AN ELECTRONICALLY FOCUSED D.C. PUMPINTERACTION STRUCTURE POSITIONED BETWEEN SAID ELECTRON GUN AND SAIDCOLLECTOR AND HAVING A PLURALITY OF ELECTROSTATIC FOCUSING LENSESPERIODICALLY SPACED LONGITUDINALLY ALONG THE PATH OF A SHEET ELECTRONBEAM DEVELOPED BETWEEN SAID ELECTRON GUN AND SAID COLLECTOR, THE LENSSTRENGTH OF SAID PLURALITY OF ELECTROSTATIC FOCUSING LENSES BEING ABOVETHE CRITICAL VALUE AT WHICH A SHEET